.alpha.-Methylenelactones have been the subject of intensive synthetic studies. Specifically, the .alpha.-methylene-.gamma.-butyrolactone group is an important structural feature of many sesquiterpenes of biological importance. In addition, .alpha.-methylene-.gamma.-butyrolactones are regarded as potential key monomers in both homopolymers and copolymers. Currently the cost of .alpha.-methylene-.gamma.-butyrolactone is too high to warrant commercial production of the resulting polymers. Some of the current synthetic routes suffer from low yields, byproducts formation and expensive starting materials. The instant invention has overcome these problems and high yields of .alpha.-methylene-.gamma.-butyrolactone are obtained by heating .gamma.-butyrolactone and paraformaldehyde in the presence of a base such as cesium carbonate or potassium carbonate.
An early synthesis of .alpha.-methylene-.gamma.-butyrolactone involved two steps (Martin et al., J. Chem. Soc. D 1:27 (1970)). The first is carboxylation of .gamma.-butyrolactone with methyl methoxymagnesium carbonate (Stiles' reagent) to produce the acid. Next, the acid is briefly treated with a mixture of aqueous formaldehyde and diethylamine, followed by a separate treatment of the crude product with sodium acetate in acetic acid. The first step requires 6-7 hours and affords almost quantitative yields, whereas the second step can be accomplished in less than 30 minutes but with yields of only 50%.
Murray et al. (Synthesis 1:35-38 (1985); see also U.S. Pat. No. 5,166,357) disclose a route to .alpha.-methylene-.gamma.-butyrolactone that also involves a two-step sequence consisting of the reaction of .gamma.-butyrolactone with ethyl formate in the presence of base, followed by refluxing the resulting .alpha.-formyl-.gamma.-butyrolactone sodium salt under nitrogen with paraformaldehyde in tetrahydrofuran. Distillation affords the desired .alpha.-methylene-.gamma.-butyrolactone as a colorless oil. This reaction sequence can best be explained by formyl transfer from carbon to oxygen followed by elimination of carboxylate anion.
Essentially all approaches to .alpha.-methylene-.gamma.-butyrolactone are liquid-phase processes. One exception is the vapor-phase process described in JP 10120672. Production of .alpha.-methylene-.gamma.-butyrolactone comprises subjecting .gamma.-butyrolactone or an alkyl-substituted .gamma.-butyrolactone, in which one or more hydrogen atoms at the .beta.- or .gamma.-position of the .gamma.-butyrolactone are substituted with C.sub.1 -C.sub.18 alkyl groups, to a gaseous phase catalytic reaction using a raw material gas containing formaldehyde or its derivative in the presence of a catalyst. Molecular oxygen is preferably added to the raw material gas and the catalyst is preferably silica alumina catalyst. Specifically, a gaseous mixture of .gamma.-butyrolactone, formaldehyde, water, nitrogen and oxygen was passed through a reactor packed with Wakogel C-200, pretreated with an aqueous potassium hydroxide solution and heated, at 330.degree. C., to afford .alpha.-methylene-.gamma.-butyrolactone at a conversion of 35.5% and a selectivity of 46.9%.
Although the above methods for the production of .alpha.-methylene-.gamma.-butyrolactone are useful, they are time consuming and are multipart processes. The present method represents an advance in the art by offering a process that is a single step, run at low temperature with high yields and good selectivity.